Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	202	196
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (19,766)	$ (29,116)
12 Months or Longer Unrealized Losses	(225,014)	(93,742)
Total Unrealized Losses	(244,780)	(122,858)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	562,625	2,119,812
12 Months or Longer Fair Value	4,027,690	3,052,744
Total Fair Value	4,590,315	5,172,556
U.S. Treasury securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(994)
12 Months or Longer Unrealized Losses	(19,535)	(11,127)
Total Unrealized Losses	(19,535)	(12,121)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		48,182
12 Months or Longer Fair Value	383,890	344,073
Total Fair Value	383,890	392,255
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(967)	(642)
12 Months or Longer Unrealized Losses	(11,532)	(6,469)
Total Unrealized Losses	(12,499)	(7,111)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	34,033	59,358
12 Months or Longer Fair Value	203,184	183,243
Total Fair Value	237,217	242,601
Government agency mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(11,097)	(976)
12 Months or Longer Unrealized Losses	(9,383)	(4,492)
Total Unrealized Losses	(20,480)	(5,468)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	291,901	200,963
12 Months or Longer Fair Value	127,392	150,427
Total Fair Value	419,293	351,390
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1)	(1)
12 Months or Longer Unrealized Losses	(7,934)	(4,129)
Total Unrealized Losses	(7,935)	(4,130)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	181	63
12 Months or Longer Fair Value	148,966	168,342
Total Fair Value	149,147	168,405
Collateralized mortgage obligations: Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,166)	(23,236)
12 Months or Longer Unrealized Losses	(151,830)	(53,510)
Total Unrealized Losses	(152,996)	(76,746)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	44,805	1,473,170
12 Months or Longer Fair Value	2,709,161	1,803,338
Total Fair Value	2,753,966	3,276,508
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(6,167)	(3,203)
12 Months or Longer Unrealized Losses	(24,387)	(14,015)
Total Unrealized Losses	(30,554)	(17,218)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	182,303	327,435
12 Months or Longer Fair Value	444,936	403,321
Total Fair Value	627,239	730,756
States and political subdivisions
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(368)	(64)
12 Months or Longer Unrealized Losses	(413)
Total Unrealized Losses	(781)	(64)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	9,402	10,641
12 Months or Longer Fair Value	10,161
Total Fair Value	$ 19,563	$ 10,641